Integrated circuits are continually being built and interconnected in denser and more complex packages. These circuits are being used to address high technology electronic applications and often include circuits that are designed to operate at high frequencies and at relatively high power levels. For example, many high performance digital computers are being implemented using VLSI (very large scale integration) ECL (emitter coupled logic) circuits operating at frequencies approaching 400 mega-Hertz and power density levels approaching 30 Watts per square centimeter.
When used in a relatively dense electronic package, this type of circuitry must meet rigidly defined electrical and mechanical specifications. These specifications concern line impedance, continuity, and minimal noise and cross-talk interference. Unfortunately, these design criteria have been met with only limited success in the prior art. For example, prior art structures, which have been designed to interconnect components of high performance digital computers, have included multiple layers consisting of conductors disposed in a polymer matrix. Those prior art structures have dielectric properties that allow high speed pulse propagation, but fail to significantly reduce the cross-talk of high speed signals in a highly dense electronic package.
Other types of prior art structures have been designed to attempt to overcome the cross-talk problem referred to above and to improve the line impedance characteristic throughout the structure. Typically, this includes particularly selected shapes, materials, widths and thicknesses to implement the various planes comprising the structure. These types of prior art structures, however, have been unable to provide adequate high frequency and controlled impedance operation while sufficiently minimizing cross-talk.
These shortcomings are especially prevalent in structures using an organic dielectric material, such as polyimide, adjacent an interconnect or a transmission line. Such a dielectric material requires venting during fabrication of the structure to release moisture in the dielectric material which could cause delamination. Thus, the use of a surface layer covering the dielectric material must be properly vented. Unfortunately, venting is difficult because wherever an interconnect or transmission line crosses a ventilation hole, a discontinuity ensues which increases the impedance and cross-talk between signals carried by the interconnect or transmission line.